Audio bandwidth interface apparatus for pilot wire relays

Abstract

In a pilot wire relay electrical power distribution protection system, an audio band interface apparatus acquires a Hz pilot wire signal from the pilot wire relay and produces signals having frequencies within the frequency band pass of fibre optic communication channels. The apparatus enables pilot relays, which previously communicated over hard wire connections such as dedicated telephone lines, to communicate through a fibre optic communication channel without the necessity for extensive modifications to the pilot wire relay.

We claim: 
     
       1. An audio interface apparatus for use with a protective relay for an electrical power distribution system, said apparatus comprising: a. means for receiving an output signal from said protective relay, which output signal is representative of line current conditions at a predetermined location in said electrical power distribution system;   b. means for converting said protective relay output signal into a first square wave signal;   c. means for generating a first local signal having a first frequency within the audio band;   d. means for generating a local audio bandwidth output signal for transmission to at least one remote audio bandwidth interface apparatus over an audio band communication channel, said local audio bandwidth output signal comprising said first local signal modulated by said first square wave;   e. means for receiving, from said audio band communication channel, a remote signal comprising the local audio bandwidth output signal generated by at least one remote audio bandwidth interface apparatus;   f. means for converting said remote signal into a second square wave signal;   g. means for comparing the phase of said first square wave signal with the phase of said second square wave signal and generating a trip output signal when said phase comparison exceeds predetermined limits; and   h. means for detecting faults in said audio band communication channel.   
     
     
       2. The audio bandwidth interface apparatus in accordance with claim 1 wherein said means for detecting faults in said audio band communication channel comprises: a. means for generating a second local signal having a second frequency within the audio band;   b. means for mixing said second local signal into said local audio bandwidth output signal;   c. means for detecting the absence of said second local signal in said received remote signal; and   d. selectively generating a trip output signal when the absence of said second local signal from said received remote signal exceeds a predetermined period of time.